1. Field of the Invention
The present invention relates to a diecasting machine utilizing a hybrid hydraulic circuit.
2. Description of the Related Art
A diecasting machine is an apparatus in which a piston of an injection cylinder is actuated by hydraulic pressure to inject and load molten metal fed to a molten metal loading sleeve into a clamped mold at high speed and, after dwelling/cooling is performed at high pressure, the mold is opened to remove the diecast product. In the high-speed injection/loading operation, a large amount of hydraulic fluid need be supplied quickly to the injection cylinder to move the piston at high speed. In the dwelling/cooling operation (particularly in the dwelling operation), high pressure is necessary for gradually supplying molten metal as the volume of the molten metal loaded in the mold decreases due to the cooling.
For this reason, as shown in FIG. 3, a typical prior-art diecasting machine B comprises a single hydraulic pump (not shown), a motor (not shown) for driving the pump, and an accumulator 53 for storing a large amount of hydraulic fluid under high pressure and for quickly supplying the hydraulic fluid to an injection cylinder 52 in injecting and loading molten metal at high speed.
However, the hydraulic circuit (not shown) using the accumulator 53 is extremely complicated and requires many hydraulic control valves (not shown) and long hydraulic fluid piping (not shown). Further, the amount of hydraulic fluid to be used and energy loss is large, and the injection accuracy is not satisfactory.
To solve such problems, a method of controlling diecast injection is proposed which does not employ such a hydraulic circuit but employs a ball thread driven and controlled by a servomotor (as disclosed in Patent Number JP10202354, which is hereinafter referred to as the first prior art method). In this first prior art method, since the injection pump is driven by a highly controllable electric servomotor, not by a hydraulic cylinder, the injection speed can be easily varied as desired depending on the configuration of the mold cavity. Further, the inclusion of air hardly occurs, and the surface rise in the cavity can be controlled with good repeatability.
Further, since the servo-control is employed, the completion of loading of molten metal into the mold cavity can be detected by the load torque applied to the servomotor. After the completion of loading is detected, control over the servomotor is switched from the rotational speed control to the torque control. In this state, the pressure can be set to a desired value with good repeatability, so that uniform and stable diecasting products can be produced. In this way, in this first prior art method, the injection speed and the injection pressure can be set as desired with better repeatability, as compared with hydraulic driving or pneumatic driving. Therefore, high-quality diecasting products with little difference in quality can be advantageously obtained.
However, the use of a ball thread as the driving source for injection makes is difficult to apply this method to a large diecasting machine.
In diecasting, a large amount of hydraulic fluid need be supplied to the injection cylinder in injecting molten metal, whereas not a large amount of hydraulic fluid supply but a high pressure is required in dwelling. Therefore, another prior art proposes that the speed control in injecting molten metal be switched to the pressure control in dwelling electrically using a single hydraulic control valve (as disclosed in Patent Number JP56159136, which is hereinafter referred to as the second prior art method). However, this second prior art method requires a complicated hydraulic control valve and also requires the use of a hydraulic pump capable of realizing the maximum discharge rate demanded by the diecasting machine. Therefore, a relatively large hydraulic pump need be used, hydraulic fluid to be used cannot be saved and energy loss occurs.
Still another prior art (disclosed in Patent Number JP2000033472, which is hereinafter referred to as the third prior art method) proposes a diecasting machine employing a flywheel with a clutch for saving energy. In this third prior art method, the flywheel is constantly rotated by an electric injection servomotor, and the clutch is connected at the timing of power supply in the high-speed injection operation and the pressurizing/dwelling operation. In this method, however, the flywheel need be constantly rotated at high speed even when the high-speed injection operation or the pressurizing/dwelling operation is not performed. Therefore, although a smaller electric servomotor for injection can be used, energy loss cannot be avoided. Moreover, this method requires a complicated mechanism such as a clutch as well as an electric control circuit.